Hitherto, a color processing apparatus of a color image in which color image information is input and subjected to the color correcting process and then output to a color printer has been used.
In such a kind of color processing apparatus, since the printing ink or toner is not completely cyan, magenta, and yellow, the color correcting process is generally performed by way of a linear masking method.
FIG. 3 shows a conventional color processing apparatus using the linear masking method. A color image to be read is color separated and read out by a readout unit 1 and converted into density signals Y, M, and C of yellow, magenta, and cyan from densities Dr, Dg, and Db of the red, green, and blue light. These density signals are sent to a color processing apparatus 11. In the color processing apparatus 11, the input color signals Y, M, and C are converted into the desired color signals Y', M', and C' according to the characteristics of the printing ink or toner on the basis of the following density expressions called linear masking equations and sent to various color printers (output unit) 13 such as ink jet color printer, thermal transfer copying color printer, electrophotographic color printer, and the like. EQU Y'=a .sub.1 Y-a.sub.2 M-a.sub.3 C (1) EQU M'=-a.sub.4 Y+a.sub.5 M-a.sub.6 C (2) EQU C'=-a.sub.7 Y-a.sub.8 M+a.sub.9 C (3)
The output unit 13 prints color inks in yellow, magenta, and cyan in accordance with the signals (signals which were subjected to the masking process) Y', M', and C' after they were subjected to the color correcting process, respectively, thereby reproducing a color image on a recording medium.
The color component of each printing ink or toner of yellow, magenta, and cyan actually includes the other color components. Therefore, the correction coefficients a.sub.1 to a.sub.9 of the expressions (1) to (3) are set to the proper values in accordance with the characteristics of the ink or toner and the color component is corrected.
For example, a.sub.2 in the expression (1) is the coefficient to correct the yellow component contained in the magenta ink or toner and a.sub.3 is the coefficient to correct the yellow component contained in the cyan ink or toner.
Such a linear masking method can be realized by a simple circuit arrangement and the coefficients can be easily optimized by the computer simulation; therefore, this method is widely used.
However, in general, the output characteristics in the actual color printers such as ink jet color printer, thermal transfer copying color printer, electrophotographic color printer, and the like are not linear and the characteristics in the case of mixed colors are further complicated nonlinear characteristics.
Therefore, according to the linear masking method, the color correcting processes which can sufficiently correct the color characteristics of the printing ink or toner of the printer cannot be realized. This causes the problem that the color difference between the original image and the output reproduced image is small with respect to certain colors but is large with respect to other colors.
To avoid such problems, a nonlinear masking method whereby the color processes are executed by quadratic masking equations is also proposed. However, the circuit constitution and simulation are complicated even with this masking method, and it is difficult to completely correct the complicated color characteristics of the printer.
On the other hand, as a method of perfectly correcting the color characteristics of the printer, a color correcting method whereby one set of outputs of Y', M', and C' are made to correspond to each combination of the densities of the input data of three colors of Y, M, and C is known.
FIG. 4 shows a color conversion memory according to the above method. The digital color density signals Y, M, and C of three colors of yellow, magenta, and cyan which are input from the readout unit 1 are input as the address data of a color conversion memory 15. The conversion data Y', M', and C' of three colors which have been preliminarily stored in a table in the memory 15 are read out on the basis of the address data and output to the output unit 13. The output unit 13 prints the color images of yellow, magenta, and cyan on the recording medium in correspondence to the conversion data Y', M', and C'.
According to this conversion data storing method, since the input data and output data are made to perfectly correspond in a one-to-one correspondence manner, the color processes which can completely correct the printer characteristics can be theoretically realized.
However, the above conversion data storing method has a serious drawback such that the necessary memory capacity is extremely large. Namely, assuming that each of the input digital signals Y, M, and C of the respective colors consists of m bits, only 2.sup.m states are provided for each color, so that the number of states which can be expressed by synthesizing three colors will be 2.sup.3m. On the other hand, assuming that each of the output signals Y', M', and C' of the respective colors also consists of m bits, 2.sup.3m bits are necessary as addresses and 3 m bits are needed as data for the color conversion memory 15. Therefore, 23 m .times.3 m bits are necessary as the whole memory capacity.
For example, when m=6, (2.sup.3m .times.3 m=2.sup.18 .times.18=) 4,718,592 bits are needed. When m=8, (2.sup.3m .times.3m=2.sup.24 .times.24=) 402,653,184 bits are needed. Accordingly, the manufacturing cost of the apparatus becomes very expensive. There is also a problem such that a very large amount of work is required to calculate the data to be stored in the color conversion memory 15 by the simulation.
The ink jet color printer will be further explained in detail as an example.
FIG. 5 shows a method of scanning ink jet heads for obtaining color images by overlapping the inks of three colors; yellow, magenta, and cyan.
In the diagram, reference numerals 101a to 101c denote multi nozzle heads which are arranged with a distance d held, respectively. Each head is scanned on a recording paper 103 in the direction indicated by an arrow 104 at a speed of v while emitting the ink from an orifice 102. The head 101a is used for the yellow ink. The head 101b is used for the magenta ink. The head 101c is used for the cyan ink. The yellow, magenta, and cyan inks are printed on the recording paper 103 in accordance with this order.
FIG. 6 is a block diagram for the image signal processed by such an ink jet recording apparatus. Input signals 105a to 105c indicative of the image densities of yellow, magenta, and cyan are input to a color processing unit 106 and subjected to the color processes such as a masking process and the like. Thereafter, the processed signals are input to a gradation correcting unit 107 and are .gamma. corrected. The yellow signal among the three corrected color signals is directly sent to a recording head 109a. However, the magenta and cyan signals are temporarily stored into buffers 108a and 108b, respectively, and thereafter, they are delayed by the times corresponding to the distance d in the scanning direction of the recording head, namely, by the time of d/v in the case of the magenta signal and by the time of 2d/v in the case of the cyan signal and sent to heads 109b and 109c. Thus, the respective color inks of yellow, magenta, and cyan are printed at the same position on the recording paper 103 and the color image is reproduced.
The .gamma. correction in the gradation correcting unit 107 is performed so as to obtain the linear relation between the input image density signal and the density of the printed image with respect to each color of yellow, magenta, and cyan. The .gamma. characteristics after the correction become as shown in FIG. 7 with regard to yellow, magenta, and cyan.
However, these .gamma. characteristics are obtained when the image is printed in single color of each of yellow, magenta, and cyan. The .gamma. characteristics differ in the case of the mixture of two or three colors.
In the case of the mixture, the .gamma. characteristic of each color component depends on an amount of ink printed previously.
FIG. 8 shows a change in .gamma. characteristic of magenta by the amount of yellow ink printed previously.
In FIG. 8, numeral 110a denotes .gamma. characteristic of magenta in the case where no yellow image is printed but the magenta image was first printed. With an increase in yellow print amount, the .gamma. characteristic of magenta changes as shown at 11Ob to 11Od.
FIG. 8 shows the relationship between a driving signal and image density in a situation where one is printed prior to the printing of another color. The driving signal performs the printing of the other color, and the image density is of an image actually reproduced on the paper.
IN this situation, the second color that is printed could be of any color ink (yellow, magenta or cyan). If the printing order of the other color is the second of thereafter (i.e., the other color is printed over the same area where printing of first color occurred), the characteristics of the other color are expressed in FIG. 8.
It is considered that such a phenomenon is caused by the nonlinear mechanism when the ink is absorbed by the paper. However, there is the nonlinear relation between the output image density signal and the color component of the output image because of this phenomenon. Therefore, there is a drawback, that sufficient color reproduction cannot be derived by the linear color correcting processes such as a linear masking method or the like. For example, in the ordinary linear masking method, assuming that the input yellow, magenta, and cyan signals are respectively Y, M, and C, the following conversion is performed. ##EQU1## However, according to this method, since there is the linear relation between the input signals and the output signals, it is impossible to correct the printer characteristics which nonlinearly change in accordance with an amount of ink printed previously.
To solve this problem, a method whereby the nonlinear color correction is performed using the masking equations of two or higher order is also proposed. This method, however, has the inconvenience such that the circuit constitution is complicated and expensive.
On the other hand, in the case of using a method whereby tables in the memory are referred to with respect to all of the color correcting processes, there is the inconvenience that a very large amount of memory capacity is necessary as mentioned above.